Screw pumps are widely used in industrial processes to provide a clean and/or low pressure environment for the manufacture of products. Applications include the pharmaceutical and semiconductor manufacturing industries. A typical screw pump mechanism comprises two spaced parallel shafts each carrying externally threaded rotors, the shafts being mounted in a pump body such that the threads of the rotors intermesh. Close tolerances between the rotor threads at the points of intermeshing and with the internal surface of the pump body (which acts as a stator) cause volumes of gas entering at an inlet to be trapped between the threads of the rotors and the internal surface and thereby urged towards an outlet of the pump as the rotors rotate.
During use, heat is generated as a result of the compression of the gas by the rotors acting in combination with one another. Consequently, the temperature of the rotors rapidly rises. By comparison, the bulk of the stator is large and heating thereof is somewhat slower. This produces a disparity in temperature between the rotors and the stator which, if allowed to build up unabated, could result in the rotors seizing within the stator as the clearance therebetween is reduced. Therefore, it is desirable to provide a system for cooling the rotors.
FIG. 1 illustrates schematically one known arrangement for cooling an outlet section of a double-ended rotor of a screw pump, as illustrated in our earlier International patent application no. WO 2004/036049, the contents of which are incorporated herein by reference. In this arrangement, a central cavity 10 is formed in each end of the threaded body 12 of the rotor (one end only shown in FIG. 1), the cavity 10 being co-axial with the body 12, the longitudinal axis of which is indicated at 14. A shaft 16 is attached to the body 12 by means of bolts 18 such that the shaft 16 extends into the cavity 10 and rotates with the body 12 of the rotor during use. The shaft 16 has a first central bore 20 formed therein. The first bore 20 houses a coolant supply tube 22 for supplying coolant pumped from a source thereof into a second central bore 24 of the shaft 16, the second bore 24 being co-axial with the first bore 20. The coolant flows from the second bore 24 into the cavity 10, wherein the coolant flows radially outwards between the end 26 of the shaft 16 and the end wall 28 of the cavity 10, and then flows away from the end wall 28 within a narrow annular gap 30 located between the cylindrical wall 32 of the shaft 16 and the cylindrical wall 34 of the cavity 10. Radial bores 36 formed in the shaft 16 allow the coolant to flow into the first bore 20 of the shaft 16 and back towards the end 38 of the shaft 16, from which it is discharged into a reservoir (not shown) with a pumping mechanism for returning the coolant to the supply tube 22.
It is an aim of at least the preferred embodiment of the invention to provide an improved arrangement for cooling the rotor of a screw pump.